As usual, it depends.

It depends on what the terms of employment are. Typically, any code you write for your employer while you are working on the employer's systems belong to your employer. Code you write at home on your own equipment belongs to you -- although I've heard of contracts where that isn't the case.

To use your analogy of a craftsman, a master carpenter may work at a job where he decides he needs to build a jig, or a prototype. When it's finished, it's used for the project. But the master carpenter doesn't *own* the jig -- its creation was paid for my the owner of the project. He can't take it home, or use on another job. However, if he goes home and builds an identical jig, that's his to keep and use on any project he sees fit.

In the software world, this can be a matter of negotiation -- putting value on the tools that might get developed during the project that the develop might like to take away. If everything needs to be left behind, then the price would be higher than if the consultant is able to take away some modules that are a) not necessarily proprietary and b) useful in a variety of applications.

A consultant who obfuscates a customer's code, or does anything else to hold the customer's code hostage might provide a win in the short term, but it will almost certainly result is a loss later on, when no one will want to hire the consultant again. It also goes against the nature of Open Source software, which I see as a meritocracy, my preferred type of society.